marginal rays

Definition: Light which passes through an optical system away from the optical axis, towards the edge of the lens aperture. * It crosses the optical axis at the image plane.
Misleading names
Marginal in name but not in effect: while these rays may appear to be dismissed, they are the ones responsible for many of the image quality problems we have to deal with. Marginal rays become more of a problem as the field angle increases.

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